I really like the Greasemonkey extension for Firefox, which lets you change the appearance and functionality of web sites using snippets of DHTML. But some site developers are a little worried about it, since it goes beyond adblockers, popup blockers and other client-side tools and literally lets you completely rewrite the sites you visit. So, I'm not surprised that Google has started warning some Gmail users that Greasemonkey "interferes with Gmail," and should be disabled when users browse the site. I've used a couple of Greasemonkey hacks for Gmail — to add a Delete button and save searches — and they go a long way toward adding functionality that Google has left out. But I can't really blame Google for trying, especially if the extensions are hitting their servers hard. In the end, however, commercial site operators are going to have to learn to cope with Greasemonkey in less aggressive ways, or risk alienating their customers.